Thank you everybody for great responses. Let me add one more information.
Over the weekend, my audiophile buddy visited me. He came with a tube pre (forgot the brand) and DIY tube amp. The tube pre/power added a significant amount of texture to the sound and the thin sound became pretty thick. However, both of us agreed that the thicker sound was not as accurate as the sound A300 made. Pleasing for sure, but I didn't think I wanted to replace A300 with the tube combo. (Of course expensive tube combos may change my mind...) The tubes made "extreme tube sounds" to quote my friend and the tightness of the base mostly disappeared. (Mind you, A300 does not control base that well, but the tubes were a lot worse) My friend's suggestion was to add a little bit of tube touch, e.g. tube CD player to add texture to the thin sound.
It seems that the CDM 9NTs can make thicker sound, although they tend to feel like a little bit bright (due to metal tweeter and non-rolling high frequency response). A300 is very good amp for the money, smooth, muscal, warm, and relatively neutral, but as Bunker suggested can sound thin. These statements are based on my experiments and I may be wrong.
Please share your experience with A300/A308, or other MF amps!